1. Field of the Invention
The present invention relates to a magnetic marker attached to a good for detecting the existence of the magnetic marker.
2. Description of the Prior Art
It is known to attach markers to goods to detect a quantity and kind of goods or to prevent theft. Such markers are attached to a good so that the marker cannot be noticed readily, and they are detected by using magnetic properties or microwaves.
There are various kinds of such markers. For example, if an amorphous thin ribbon or thin wire marker is subjected to an AC magnetic field, disturbances in the magnetic field of a scan area or harmonic components of an output pulse from the magnetic field can be detected. Another example of a marker is one which comprises a coil and a capacitor made of aluminum which is subjected to radiation or electric waves, thus enabling LC resonance detection. Among the markers, there is a magnetic marker having large Barkhausen characteristic, and sharp pulses generated on magnetization reversal can be detected from an AC magnetic field. This marker has the advantages of having a high sensitivity, a light weight and less erroneous detections.
Large Barkhausen reversal is a phenomenon caused by the movement of magnetic domains in a material, and it occurs when a limit magnetic filed H* needed to generate inverse magnetic domains is larger than a minimum magnetic filed H.sub.O needed to move magnetic domains. Inverse magnetic domains are formed when an effective magnetic field H.sub.eff, which is equal to an external magnetic field H.sub.ex substrated by a demagnetizing field H.sub.d generated at the magnetic thin wire by the external magnetic filed H.sub.ex, exceeds the limit magnetic field H*. The inverse magnetic domains, upon formation, instantly move to generate a sharp magnetization reversal. It is characteristic of a large Barkhausen reversal that an output induced voltage accompanied by the magnetization inversion is constant irrespective of either the external magnetic field or a speed of change in magnetic field, and that a sharp pulse waveform having high harmonic components is present.
Among such magnetic markers, a marker disclosed in Japanese Patent laid open Publication 4-195384/1992 has a structure in which soft magnetic materials having a low coercive force are arranged at two ends of a magnetic thin wire for generating pulses. The magnetic thin wire displays a large Barkhausen effect, and the two soft magnetic materials have a coercive force H.sub.c that is smaller than that of the magnetic thin wire. The demagnetizing field of the magnetic thin wire for generating pulses is reduced by arranging the soft magnetic materials as being close to the magnetic bar. As a result, the magnetic marker can be made compact.
Because the magnetic thin wire of the magnetic marker has a diameter of 120 .mu.m, if the length of the magnetic thin wire is as short as 50 mm or less, a good large Barkhausen effect cannot be generated, and a practically large output voltage cannot be obtained. However, it is desirable to have a magnetic marker with a shorter length to make it more compact.